1. Field of the Invention
The present invention relates to a cleaning device employing an electrostatic cleaning method; an image forming apparatus containing the cleaning device; and a process cartridge containing the cleaning device.
2. Description of the Related Art
In recent years, electrophotographic image forming apparatuses, which are employed, for example, in copiers, printers, plotters, facsimiles and complex machines having the functions thereof, have been increasingly being required to form an image of higher quality. In view of this, toner particles (i.e., coloring powder) used for image formation tend to have a smaller particle diameter. Also, in an attempt to reduce the production cost for toner and to increase the transfer rate thereof, some commercially available image forming apparatuses employ, rather than pulverized toner particles, spherical toner particles produced through, for example, the polymerization method. Meanwhile, in order to remove toner particles remaining after image formation on the surface of an image bearing member (e.g., a photoconductor), a blade cleaning method has conventionally been employed in many cases. The blade cleaning method removes toner particles by bringing a rubber blade into contact with the photoconductor surface. When the blade insufficiently comes into contact with the photoconductor surface, the spherical toner particles run through the contact portion therebetween, resulting in that the cleaning performance is prone to degrade.
When pressed against the photoconductor surface at a high contact pressure to avoid the above unfavorable phenomenon, the blade is undesirably warped to cause cleaning failures such as formation of streaky or belt-like scratches, making it difficult to stably attain desired cleaning performance. Such spherical toner particles can be cleaned when the linear pressure is considerably increased. But, in this case, the service life of an image forming apparatus using them becomes very short due to, for example, abrasion of a photoconductor drum and a cleaning blade. Further, as has been well known, spherical toner particles excellent in transferability are less cleaned with a blade than are pulverized (amorphous) toner particles.
Separately, there is known a brush cleaning method which reduces abrasion of the photoconductor surface and reliably removes spherical toner particles having a small particle diameter. The brush cleaning method is performed based, for example, on the following configuration: a brush roller is disposed so as to slidably contact with the photoconductor surface; a recovering roller is disposed so as to be in contact with the brush roller; and a rubber blade or another unit is disposed so as to remove toner particles from the recovering roller. In this configuration, cleaning is performed utilizing electrostatic force brought by applying a voltage to the recovering roller or both the recovering roller and the brush roller, which is advantageous in use of spherical toner particles, etc.
As has been well known, the voltage applied in a common transfer step has an opposite polarity to that of the toner after development and thus, the remaining toner particles on the photoconductor surface after transfer are a mixture of toner particles having the same polarity as those after development, toner particles having an opposite polarity to them, and toner particles having no polarity.
Various attempts have been made to clean such a mixture that contains positively-charged, negatively-charged, and non-charged toner particles.
For example, Japanese Patent Application Laid-Open (JP-A) No. 2002-202702 discloses an image forming device including a conductive cleaning blade positively (negatively) charged and a conductive cleaning roller negatively (positively) charged, the conductive cleaning blade being disposed upstream of the conductive cleaning roller in a direction in which a photoconductor is rotated, wherein the conductive cleaning roller charges the remaining toner particles, which have not been removed with the conductive cleaning blade, so as to have a single polarity, and then cleans them.
Japanese Patent (JP-B) No. 3994974 discloses an image forming apparatus including a conductive rotating member which is driven so as to rotate with being in contact with a surface of an image bearing member; a conductive member which is in contact with the image bearing member at a position upstream of the rotating member in a direction in which the image bearing member is moved; and a single DC constant current power source bound to one of the conductive rotating member and the conductive member, the other being connected to the ground. In this image forming apparatus, the single DC constant current power source generates a DC electric current flowing between the conductive rotating member and the conductive member via the image bearing member, forming a first electrical field, between the conductive rotating member and the image bearing member, which acts in a direction in which regularly charged toner particles are adsorbed on the conductive rotating member; and a second electrical field, between the conductive member and the image bearing member, which acts in a direction in which regularly charged toner particles are adsorbed on the image bearing member.
JP-B No. 3466825 discloses an image forming apparatus including a first image bearing member, a second image bearing member, a third image bearing member, a transfer unit, and a toner-recovering unit, wherein a toner image formed on the first image bearing member is primarily transferred onto the second image bearing member at a primary transfer portion between the first and second image bearing members; the transfer unit is configured to secondarily transfer the toner image from the second image bearing member onto the third image bearing member; and the toner-recovering unit is configured to contact-charging the toner particles remaining after secondary transfer on the second image bearing member. In this image forming apparatus, the remaining toner particles charged by the toner-recovering unit are recovered from the primary transfer portion to the image bearing member. Here, the toner-recovering unit includes a cleaning charging member and a charging bias power source for applying a voltage to the cleaning charging member, wherein the charging bias power source applies, to the cleaning charging member, a voltage obtained by superimposing an AC voltage on a DC voltage. Also, the DC voltage is constant-current controlled, and the AC voltage is constant-voltage controlled.
JP-A No. 2005-265907 discloses a cleaning method including controlling, through corotron charging which applies a voltage to a corona charger, the charge amount of toner to be cleaned; and cleaning positively and negatively charged toner particles using two brushes arranged in a row, one of the two brushes being positively charged for cleaning the negatively charged toner particles and the other being negatively charged for cleaning the positively charged toner particles.
The method disclosed in JP-A No. 2005-265907 can satisfactorily clean a mixture of positively-charged, negatively-charged, and non-charged toner particles. However, such a configuration that two brushes are arranged so as to face a photoconductor and toner-recovering devices for recovering toner particles adhering to the brushes are disposed cannot easily attain downsizing of image forming apparatuses.